1. Field of the Disclosure
The present disclosure relates to a rotatable connector that is incorporated in an automobile steering system for electrical connection of an airbag system. More particularly, the disclosure relates to a rotatable connector in which three or more flat cables are received in an annular space defined between a stator member (hereinafter “stator”) and a rotor member (hereinafter “rotor”) in a state that the winding direction of each flat cable is reversed via a reversed portion.
2. Description of the Related Art
In a conventional rotatable connector, a rotor having an inner cylindrical portion is rotatably supported by a stator having an outer cylindrical portion. In the rotatable connector, flat cables are wound and received in an annular space between the outer cylindrical portion and the inner cylindrical portion. The rotatable connector is used as means for providing electrical connection to an airbag system, an inflator, or the like that are built in a steering wheel having a limited number of rotations, which is incorporated in an automobile steering system. The flat cables are made of a band-shaped member having a conductor contained in an insulating film. Two types of rotatable connectors are known. One is wound in a spiral form and the other is wound in a halfway reversed form. The reversed type rotatable connecter, can be made substantially shorter than the spiral form type. Therefore, the reversed type rotatable connector is widespread. The reversed type rotatable connector normally has one flat cable. Recently, a rotatable connector has been proposed in which a conductor is divided into several pieces corresponding to two or more flat cables in order to cope with a trend toward a multi-circuit configuration. Such a rotatable connector is disclosed in JP-A-10-116672, for example.
FIG. 4 is a top plan view of a known rotatable connector disclosed in JP-A-10-116672. The rotatable connector includes a stator 100 having an outer cylindrical portion 100a, a rotor 101 having an inner cylindrical portion 101a and that is rotatably supported at the central position of the stator 100. A ring-shaped holder 103 is rotatably disposed in an annular space 102 between the outer cylindrical portion 100a and the inner cylindrical portion 101a, and first and second flat cables 104 and 105 received in the annular space 102 such that a winding direction of each cable is halfway reversed. In the holder 103, a pair of stationary tubes 106 is erected, a plurality of rollers 107 are rotatably supported, and the stationary tubes 106 are opposed to each other across the holder 103 while maintaining a predetermined gap with respect to one of the rollers 107 in the circumferential direction. A first opening 108 is defined in a gap between one of the stationary tubes 106 and the roller 107 opposite the one stationary tube 106. A second opening 109 is defined in a gap between the other stationary tube 106 and the roller 107 opposite the other stationary tube 106. A width in the circumferential direction of the first opening 108 is set smaller than a width in the circumferential direction of the second opening 109.
The first flat cable 104 and the second flat cable 105 are made of a band-shaped member having a plurality of conductors contained in an insulating film such as polyethylene terephthalate (PET) or other polyester films. In the drawing, the first flat cable 104 is shaded with black and the second flat cable 105 is unshaded, for convenience. The outer end of each of the flat cables 104 and 105 is connected to a stationary joint 110 provided in the outer cylindrical portion 100a and is electrically led out from the stator 100 to the outside through the stationary joint 110. In addition, the inner end of each of the flat cables 104 and 105 is connected to a movable joint 111 and is electrically led out from the rotor 101 to the outside through the movable joint 111.
In a state in which the first flat cable 104 is disposed on the outer side, the flat cables 104 and 105 are wound around an inner peripheral wall of the outer cylindrical portion 100a in a counterclockwise direction from the stationary joint 110. Then, the flat cables 104 and 105 are branched off. Specifically, the first flat cable 104 passes through the first opening 108 with a small width, and is reversely wound in a U shape around one of the rollers 107 (this portion of the flat cable will be referred to as reversed portion 104a). Meanwhile, the second flat cable 105 passes through the second opening 109 with a large width, and is reversely wound in a U shape around another roller 107 (this portion of the flat cable will be referred to as reversed portion 105a). Then, the first and second flat cables 104 and 105 are wound around an outer peripheral wall of the inner cylindrical portion 101a in a clockwise direction such that the second flat cable 105 is disposed on the outer side. Then, the flat cables 104 and 105 are led to the movable joint 111 and are received in the annular space 102.
In the rotatable connector having such a configuration, when the rotor 101 rotates in a counterclockwise direction from a neutral position (direction of arrow A), the reversed portions 104a and 105a of the first and second flat cables 104 and 105 move in the arrow A direction by an amount smaller than the rotation amount of the rotor 101. Then, the holder 103 also moves in the arrow A direction accompanied by the movement of the reversed portions 104a and 105a. As a result, the flat cables 104 and 105 are led out from the outer peripheral wall of the inner cylindrical portion 100a by an amount approximately twice as long as the movement of the reversed portions. Then, the flat cables are wound around the inner peripheral wall of the outer peripheral portion 101a. In this case, the reversed portion 105a of the second flat cable 105 having a large-diameter winding portion moves faster than the reversed portion 104a of the first flat cable 104 having a small-diameter winding portion. As mentioned above, however, since the width of the first opening 108 is smaller than that of the second opening 109, the reversed portions 104a and 105a press the stationary tube 106 facing each of the openings 108 and 109, and the holder 103 receives the pressing force from the reversed portions 104a and 105a. Accordingly, the holder 103 rotates in the annular space 102 in the arrow A direction.
To the contrary, when the rotor 101 rotates in a clockwise direction from the neutral position (arrow B direction), the reversed portions 104a and 105a of the flat cables 104 and 105 move in the arrow B direction by an amount smaller than the rotation amount of the rotor 101. The holder 103 also moves in the arrow B direction accompanied by the movement of the reversed portions 104a and 105a. As a result, the flat cables 104 and 105 are led from the inner peripheral wall of the outer cylindrical portion 100a by an amount approximately twice as long as the amount of the movement of the reversed portions 104a and 105a. Then, the flat cables are wound around the outer peripheral wall of the inner cylindrical portion 101a. In this case, the reversed portion 105a of the second flat cable 105 having a large-diameter winding portion moves faster than the reversed portion 104a of the first flat cable 104 having a small-diameter winding portion. However, since the width of the first opening 108 is set smaller than that of the second opening 109, the reversed portions 104a and 105a attract the roller 107 facing each of the openings 108 and 109, and the holder 103 receives the attracting force from the reversed portions 104a and 105a. Accordingly, the holder 103 rotates in the annular space 102 in the arrow B direction.
In the known rotational connector described above, when the rotor 101 rotates in the arrow A direction and the flat cables 104 and 105 are wound around the inner peripheral side wall of the outer cylindrical portion 10a, an output force is continuously generated in the arrow F direction in FIG. 5 at a contact portion with the roller 107 of the first flat cable 104 passing through the first opening 108. Among the components of the output force F indicated by the arrow Fx and the arrow Fy, the force component in the arrow Fx direction serves as a pressing force that allows the holder 103 to rotate. Meanwhile, the force component in the arrow Fy direction serves as a force that allows the reversed portion 104a of the first flat cable 104 to be pressed toward the outer cylindrical portion 100a side. Although the detailed descriptions thereof are omitted, the same statements are applicable to the case of the second flat cable 105 passing through the second opening 109. In addition, when the normal rewinding operation is performed, the component in the arrow Fx direction is greater than the force component in the arrow Fy direction. Accordingly, when the reversed portion 104a of the first flat cable 104 passes through the first opening 108, the reversed portion 104a presses the stationary tube 106 in a rotation direction (the arrow A direction). Similarly, when the reversed portion 105a of the second flat cable 105 passes the second opening 109, the reversed portion 105a presses the stationary tube 106 in a rotation direction. Accordingly, the holder 103 receives the pressing force from the reversed portions 104a and 105a and rotates smoothly in the arrow A direction.
However, in the known rotatable connector, it is difficult to set the respective widths of the openings so that the stationary tube facing the respective openings is pressed by the reversed portions when three or more flat cables rotate in a counterclockwise direction while reversing the winding directions at three or more openings. When the rotor rotates in a clockwise direction, it is difficult to set the respective widths of the openings so that the reversed portions of the respective flat cables attract the roller facing the respective openings. In addition, depending on the respective settings on the widths of the openings, driving forces applied to the holder from the reversed portion of the respective flat cables may be interfere with each other. Accordingly, it is difficult to perform the smooth winding operation and rewinding operation.
Accordingly, when the widths of the openings are set such that the holder is only driven by the reversed portion of the flat cable passing through the small-width opening, an excessive amount of stress is applied to the reversed portion of the flat cable passing through the small-width opening. In the small-width opening, a frictional resistance may increase excessively between the flat cable and the roller. Accordingly, the force component in the arrow Fx direction shown in FIG. 5 is lowered by the frictional resistance. As a result, the force component in the arrow Fy direction may be increased greater than the force component in the arrow Fx direction. Consequently, the movement of the reversed portion of the flat cable in the arrow A direction may be interrupted in the small-width passage. Thus the holder can not be driven smoothly. In the small-width passage in which the rotational movement of the reversed portion of the flat cable is interrupted, the reversed portion is forcibly inserted into a space of the opening. As a result, the flat cable is complexly bent, curved or buckled, or the conductor carried in an insulating film of the flat cable may be broken. Specifically, when an environment temperature increases to a high temperature, such a problem may occur frequently.